1. Technical Field of the Invention
The present invention relates to sensing a temperature, and particularly to a circuit and method for temperature sensing, with self-calibration.
2. Description of the Related Art
On-chip temperature sensors have been used in a variety of applications to measure temperature. A typical on-chip temperature sensor utilizes a proportional-to-absolute-temperature (PTAT) resistive element having a resistance that is based upon its temperature, and a ring oscillator which oscillates at a frequency that is dependent upon the resistance of the PTAT resistive element. The temperature is determined by measuring the frequency of the ring oscillator and determining the temperature sensed by the PTAT resistive element based upon the measured frequency.
One shortcoming with conventional temperature sensors is that devices and/or circuitry for measuring the frequency of the ring oscillator and determining the temperature therefrom are oftentimes complex and occupy an excessive amount of silicon space. Another shortcoming associated with conventional temperature sensors utilizing PTAT resistive elements is that the resistance thereof may vary considerably due to process variations. This can be seen to adversely affect the ability to accurately measure temperature. What is needed, then, is a temperature sensing device that can accurately and efficiently measure temperature.
Embodiments of the present invention overcome shortcomings in prior temperature sensing circuitry and satisfy a significant need for an on-chip temperature sensing device that efficiently and accurately measures temperature. In a first embodiment of the present invention, the temperature sensing device includes an oscillator circuit that generates a first signal having a frequency which is dependent upon a sensed temperature. A difference circuit receives the first signal and a reference signal having a known, temperature-insensitive frequency and generates a difference signal having a number of pulses thereon corresponding to a difference in frequency between the first signal and the reference signal. One or more counter circuits receives the difference signal and offsets a predetermined temperature value corresponding to the frequency of the reference signal, to obtain an output signal indicative of the sensed temperature.
In a second embodiment of the present invention, the oscillator circuit includes a PTAT resistive element having a resistance based upon a sensed temperature. The frequency of the first signal is then based upon the resistance of the PTAT resistive element. In order to more accurately measure temperature, the temperature sensing device includes self-calibration circuitry for comparing the frequency of the first signal with the known, temperature-insensitive frequency of the reference signal and generating one or more calibration output signals having a value indicative of an amount by which the PTAT resistive element needs to be increased or decreased. For example, the one or more calibration output signals may indicate any one of several values by which the PTAT resistive element is to be increased or decreased. Based upon the value of the calibration output signal, the PTAT resistive element may be trimmed or otherwise modified accordingly.